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Old 11-22-2006, 03:34 PM   #1
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Plenum Design

My HVAC plenum is a near square metal box. I presume insulated on the inside.
It is connected of course to the Coil Box on one side.
The top has no ducts exiting. Nor does the bottom which is just a few inches over the drip pan which rests on the ceiling joists.
All the round grey vinyl covered ducts exit on the remaining 3 sides.

I am not happy with the low output in certain rooms.

I also hate the spaghetti bowl mess of ducts running everywhere and the task of climbing over and working around so many of them.

Would it have been wrong for the plenum to have a few ducts exit on top?

Would it be unwise to raise (in the next replacement the whole furnace (horizontal in attic) and likewise the input box and the new plenum and use the bottom of the new plenum for duct exit as well?

BTW are plenums made on the job site usually?
I mean do you guys cut the holes in blank plenum panels?
How do you do this if so?

Seems to me that the ideal plenum would be a large globe.
Or does it matter that there are "dead corners" inside the plenum?

any ideas appreciated.



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Old 11-22-2006, 10:28 PM   #2
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Hi winkydink

It sounds like to me that you have a typical ceiling installation. The one thing to look for on a flex duct systems are kinks in the duct. This is the most common problem with this type of duct. I personally prefer to use metal duct and a trunk system if possible. As far as the dead spaces in the plenum, what is actually going on is your trying to squeeze equal amounts of air through each duct. However with flex duct this is hard to do. There are alot of physical air propertys that have to be taken into account when you size a duct system. The one that I prefer is a 4 air exchange per hour. To do this you have to take the cubic feet of each room and divide it by 4. That will give you the needed CFM per duct. Then you size the runs to that room for the given CFM. There are other ways to calculate comfort, I think everybody in this field has there own method. But if you walk anywhere in my house, each room feels just like the one you left. There are no cold and hotspots. And with the addition of the 58MVP furnace with its variable speed blower, it does it quite well. In my upstairs is set up the same, with the same effect but is has a 58MXA with a fixed capacity blower. Let me know if you need me to clarify any of this.

Good luck


Last edited by #CARRIERMAN; 11-22-2006 at 10:31 PM.
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